1. Field of the Invention
The present invention relates to an antenna device and a wireless communication device using the antenna device and more particularly to improvements of the antenna device that can be used suitably for a portable wireless terminal.
The present application claims priority of Japanese Patent Application No. 2003-282231 filed on Jul. 30, 2003, which is hereby incorporated by reference.
2. Description of the Related Art
Many of antennas used in a wireless communication device, especially in a portable wireless terminal such as a portable cellular phone, portable information terminal, or a like in mobile communications are of a non-directivity type. The reasons are that a direction of a base station with which a portable wireless terminal communicates varies and becomes inconstant depending on a position of the portable wireless terminal or on its movement.
Conventionally, such antennas as a monopole antenna, helical antenna, inverted F-type embedded antenna, or the like is often used in a portable cellular phone as a non-directivity-type antenna.
However, improvements in performance of an antenna become necessary as demands for a speedup in data communications or for an increase in a communication distance increase. A possible method for improving the performance of an antenna is to achieve high gain by getting an antenna to have directivity. By using this method, since an effect of lowering gain in an unwanted direction of signals is also expected, improvements not only in signal receiving sensitivity but also in an SIR (Signal to Interference Ratio) are made possible.
There is also a growing demand that a portable wireless terminal deal with signals having two or more communication frequencies or signals to be received or transmitted by two or more communication methods. To meet this demand, it is necessary that a portable wireless terminal be equipped with two or more antennas capable of dealing with signals having two or more communication frequencies or signals to be communicated by two or more communication methods or with an antenna capable of dealing with signals having two or more frequencies.
Moreover, when a high-speed communication is carried out, a range of frequencies to be used has to be wide and, when two or more communication methods are used, a frequency to be used has to be changed in some cases and, therefore, a wideband antenna that can cover all ranges of frequencies is required.
In a wireless communication device, in order to control directivity in a portable wireless terminal in particular, an antenna made up of two or more antenna elements such as an array antenna is conventionally used. However, to achieve this aim, some distance between the antenna elements is needed, which, as a result, causes the antenna itself to be made larger. Also, to control antenna directivity, signal control is required in each of the two or more antenna elements, which causes communication processing to be made complicated and, at the same time, causes an increase in power consumption. Furthermore, if two or more antennas are used to carry out communications employing two or more communication frequencies and/or employing two or more communication methods, problems related to mounting of antennas such as difficulties caused by a difference in size among the antennas and/or interference among the antennas may occur.
Moreover, switches are needed to switch each of the two or more antennas and, therefore, power loss caused by the switch produces a problem, which also causes an antenna to increase in size. The antennas that can deal with signals having two or more frequencies present another problem in that frequencies to be used are limited and actually there are cases in which they have elements that resonate at each frequency.
A shape-variable antenna is disclosed in a non-patent document, IEEE International Symposium, Antennas and Propagation Society, Vol. 3, 8–13, July, 2001, pp. 654–657, “MEMS (Micro Electro Mechanical System)-Switched Reconfigurable Antenna” (William H. Weedon, et al.) in which, in order to deal with signals having two or more frequencies, four antenna elements are arranged in a 2×2 matrix form and switches are mounted so that they switch the antenna elements between electrically connected and disconnected states and so that they control change in shape of the antenna elements so as to deal with signals in two frequency bands, that is, an L band (1 GHz to 2 GHz) and an X band (8 GHz to 12.5 GHz) and in which a wide-band MEMS switch that can deal with a signal in a frequency band of 0 to 40 MHz is employed.
However, such a conventional antenna as described in the above non-patent reference has a problem. That is, though the above antenna that can deal with signals in two frequency bands is achieved by using one device, no consideration is given to directivity and, therefore, antenna directivity cannot be controlled.